HILL: GEOLOGY OF JAMAICA. 107 
Veals the fact that the rocks alluded to are clearly intrusive into the 
Overlying sedimentary section. Our personal observations enable us to 
Say positively that, while the oldest known rocks of the island are un- 
doubtedly rolled eruptive volcanic débris, no fundamental massifs of 
Plutonic crystalline rocks, granite or other kind, are known to occur as 
à previously formed basement or axis to the sedimentary section of 
Jamaica. On the other hand, these rocks, by occurrence, are all erup- 
tive and intrusive, and in age are contemporaneous in origin with the 
Sedimentary rocks. 
The igneous rocks can be classified by age, occurrence, and miner- 
Alogic composition into three distinct categories, as follows : — 
I. The andesitic (mostly hornblende) boulders, pebbles, and tuffs of 
the Blue Mountain Series, of eruptive origin from unknown vents, con- 
temporaneously deposited with the Cretaceous sediments of the lower 
beds and occurring as rolled and worked over material in the Richmond 
beds of the upper part of that series. 
U. The hornblende diorite and granitoid porphyries of the five east- 
em parishes of the island, constituting dikes and masses, or laccoliths, 
intruded through the rocks of the Blue Mountain Series and into the 
Ontpelier formation of the Oceanic Series. These rocks are of Mid- 
'ertiary age. 
III. Eruptive amygdaloidal basalts of the Low Layton stock or vol- 
Canic neck in the north coast of Portland. Mid-Tertiary. 
I. The Boulder Material of the Blue Mountain Series. — In the gen- 
eral description of the Blue Mountain Series we have noted the pre- 
Ominance of conglomerates, boulders, pebbles, and tuffs, composed of 
rolled volcanic rock, and the fact that the accompanying shales and 
Sandstones are made up almost, if not entirely, of this material, which 
198 undergone aqueous trituration. This boulder material is the oldest 
formation exposed on the island, and it has survived by rolling and re- 
deposition through all succeeding epochs, being especially conspicuous 
m the Richmond, Bowden, Kingston, and modern aggradational deposits. 
Particular attention was paid by us to the study of the composition 
And Origin of this material. Specimens from what are apparently the 
Oldest formation of the island, — the Frankenfield conglomerates of the 
tio Minho section of Clarendon, — as determined petrographically by 
ross, show that the material is almost exclusively composed of horn- 
ende-andesites and hornblende-andesite tuffs. 
Original masses in situ from which this boulder material could have 
“rived, are nowhere known to be exposed on the island, and hence its 
